[Taxacom] Invertebrate phylogeny begins to shift
kennethkinman at webtv.net
Mon Jul 18 11:02:54 CDT 2011
At last, fossil evidence is emerging to back up my contention that
brachipods did NOT evolve from folded-up halkieriids. And this has much
wider implications concerning the origins and phylogeny of other
bivalved invertebrates (bivalved arthropods, bivalved mollusks, etc.),
and by extension, the evolution of all invertebrates.
I have long argued that halkieriids no doubt evolved by the
"unfolding" of a brachiopod ancestor, thus yielding a slug-like body
with the two shells protecting vulnerable vital organs. Something
similar no doubt occurred when ostracod-like ancestors gave rise to
arthropods, and of course, most of you will probably recall my view that
Bivalvia gave rise to the other molluscs.
The old idea that worm-like forms "folded up" to create various
bivalved invertebrate groups is totally backwards and is now slowly
crumbling (and this is long overdue). The last vestiges of Vermes will
finally disappear, and a new understanding of the broad picture of
invertebrate evolution will emerge. Below is the abstract of the paper
by Holmer, et al., 2011.
HOLMER, L. E., SKOVSTED, C. B., LARSSON, C., BROCK, G. A. and ZHANG, Z.
(2011). "First record of a bivalved larval shell in Early Cambrian
tommotiids and its phylogenetic significance." Palaeontology,
Abstract. Brachiopods are marine Lophotrochozoa whose soft parts are
enclosed in a bivalved shell. Although brachiopods are represented by a
rich record from the Early Cambrian to the present, the origin of their
bivalved body plan remains controversial. The Early Cambrian
organophosphatic tommotiids Micrina and Paterimitra from Australia have
been proposed as stem brachiopods. Here, we describe their earliest
ontogeny, indicating that tommotiids possessed bivalved planktotrophic
larvae. The curious combinations of characters in Micrina and
Paterimitra indicate that they may belong to the stems of the
Linguliformea and Rhynchonelliformea, respectively. The bivalved shell
of adult living brachiopods may represent a plesiomorphic character
retained from planktic tommotiid larvae; the crown group body plan of
the Brachiopoda may have evolved through the paedomorphic retention of a
bivalved larval state.
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